These were all questions I was googling/asking when my wife came to me with a black screen inside of the camera app. I followed some black magic on youtube about toggling low power mode and hard resetting with some success but it was not consistent. Finally I found an apple forum post from a guy who went through the whole process below in iFixit after buying a new camera module:

He got in to replace the camera just to find that the camera module cable had disconnected. After a shy grin his girlfriend admitted her phone had dropped about the time the camera stopped working.

After reading this a lightbulb went off. Knowing that the kids use my wife’s iPhone to play games and watch video I knew this had to be a similar issue. I took out my trusty iFixIt tool kit and got to work. Low and behold the rear facing camera cable was loose.

I went through the iFixIt guide and then popped off the rear camera connection to make sure no lint or dirt was under the contacts and reconnected it. I put the phone back together and poof! The camera started working again like magic.